Fiber optic cable has become a preferred medium for transmitting data and telecommunications signals over long distances. Glass or plastic optical fibers serve as the transmission medium of fiber optic cable. Optical fibers are relatively thin and fragile in comparison to other media and are particularly susceptible to tensile stress, in particular, during cable processing and manufacturing. Tensile stress can change the optical characteristics of optical fibers and can adversely affect the transmission performance of a fiber optic cable.
Strength members, e.g., rigid strength rods and flexible strength tapes or yarns, as well as strength reinforcement systems, e.g., arrangements and/or combinations of rigid and/or flexible strength members, are used in cable systems. Strength members add strength and provide flexibility to a cable and its components. Flexible strength members, e.g., strength yarns, are particularly suited for adding strength to and reinforcing cable components, e.g., optical fibers. Strength yarns can help to protect optical fibers against stress caused by an excessive tensile load to a cable. Strength yarns are typically constructed of materials having sufficient strength and flexibility for responding to a tensile load. When a cable receives an excessive tensile load, strength yarns can respond to the tensile stress by twisting and undulating to thereby absorb the stress load and to help protect the optical fibers.
Strength yarns are provided in various shapes and compositions, and can be integrated into a variety of cable designs. Strength yarns can be stranded into a cable to extend linearly along a central axis of the cable and/or to wrap helically around a cable core. Strength yarns can be integrated and/or stranded with one or more components of a strength reinforcement system added to a cable to provide strength and protection. In addition, strength yarns can be stranded with and/or around optical fiber buffer tubes and extend with the optical fibers along a central axis of a fiber optic cable.
Flexible strength members and reinforcement systems are typically constructed of such materials as plastics, fiberglass reinforced plastics, polyesters, polyethylene, high strength polyethylene and aramid fibers. Many of these materials, however, are relatively expensive, e.g., high strength polyethylene or aramid fibers. In addition, many of the materials are relatively susceptible to thermal damage and generate smoke and ignite when exposed to high temperatures and/or a continuous flame, e.g., plastics, polyester and aramid fibers. These materials can render cable installations and, in particular, indoor cable of inter- and intra-building networks, vulnerable to high temperatures and fire damage. To reduce the flammability of indoor cable systems, strength materials are often coated with flame retardant compounds, e.g., fluorinated polymers and TEFLON®, which add considerable cost to cable designs.
Therefore, it is desirable to provide a strength member constructed of low cost materials for providing tensile strength and flexibility to a cable and its components. It is desirable to provide a strength member for use with a cable and its components that exhibits low flammability and low smoke generation properties.